My online friend wants to commit suicide, what do I do?

[Sara]

I know this sounds dumb. I met one of my closest friends on the Internet, through twitter. I'm the only person she truly trusts with this. She wants to for many reasons, and won't listen when I tell her I care! I can't call her mom, I can't tell anyone she knows offline, I can't do any of those things! She won't go to a school counselor, she won't tell her mother, she won't do anything to get help because she doesnt want it!
I know my ways of helping are limited, and I think I've tried all ways, which is why my question is split into two halves.
One being, how do I help? Shes talking about it again.
The other half being, if she does, what do I do to get better?